Along with the recent advance of digital technologies, a radiographic image or the like is converted into a digital image signal, which undergoes an image process such as a frequency process and the like, and the processed digital image is displayed on a CRT or the like or is printed on a film by a printer. Such frequency process is applied by decomposing an image to be processed into image components of a plurality of frequency bands, and increasing/decreasing the decomposed component values using a conversion table.
The digital image signal that has undergone the frequency process often undergoes a halftone process so as to optimally fall within the dynamic ranges of output media such as a CRT, film, and the like. For example, such process is done by adjusting the conversion table (also called a halftone conversion curve) so that the pixel value range of an object in an image falls within the dynamic range of an output medium. According to this method, since the entire object image is processed to fall within the dynamic range given to an output medium, the dynamic range of the output medium can be fully utilized, and an output image which allows easy diagnosis can be obtained. Also, since objects having different pixel value ranges are adjusted to fall within the dynamic range of an output medium, an output image having an equivalent density distribution can be obtained.
However, as the aforementioned halftone process changes the conversion table depending on the pixel value range of an object, the contrast of an image changes before and after the halftone process. More specifically, when the pixel value range of an object is narrow, since the pixel value range of an image after the halftone process is broadened, the contrast becomes high. When the pixel value range of an object is broad, since the pixel value range of an image after the halftone process is narrowed, the contrast becomes short.